1. Field of the Invention
This invention relates to a method of acidizing wells, and more particularly to a method of acidizing which provides proper diversion of the acidizing solution into a heterogeneous subterranean formation penetrated by a well.
2. Description of the Prior Art
Many reservoirs which require workover operations or stimulation do not have uniform permeability throughout but instead exhibit heterogeneity. By heterogeneity, it is meant that the reservoir is comprised of stratified layers of varying permeability, or that it contains fractures, cracks, fissures, streaks, vuggs, or zones of varying permeability that cause injected fluids to advance through the reservoir nonuniformity. Thus, in treating formations that have strata or zones of different permeabilities, or which otherwise are structurally faulted so that injected treating fluid does not advance through the formation at a substantially uniform rate, it is often desirable to divert the treating fluid from the more highly permeable portions of the formation into less permeable portions of the formation. In the past, this diversion has been primarily accomplished by pumping a volume of treating fluid into the formation, then following it with a diverting agent suspended in a carrier fluid to seal off the portion of the formation where the first treating fluid penetrated, and then following the diverting agent with a second stage of treating liquid which would be forced into another portion of the formation. The carrier fluid containing the diverting agent flows most readily into the portion of the formation having the largest pores, fissures, and vuggs until the diverting agent bridges on the face of that portion of the formation and forms a relatively impermeable filter cake, thus diverting the remaining fluid to the next most permeable portion of the formation. These steps may be repeated until the desired number of stages of treating fluid have been injected. Alternatively, the diverting agent may be dispersed in the treating fluid for progressive diversion throughout the treating operation.
However, after either stage or progressive treatments, the diverting agent must be removed to allow the oil to flow into the well. This is usually accomplished by selecting diverting agents that will dissolve in the formation fluids or in the treating fluids, or diverting agents which will melt or sublime at formation temperatures.
A great variety of diverting agents have been used to plug formation openings and to divert treating liquids to other zones of a formation. Wax beads, naphthalene particles, or benzoic acid in the form of finely divided particles and as flakes, and sodium chloride, as graded rock salt, are conventionally used as diverting agents. Additionally, a number of fibrous and granulated materials such as asbestos, cotton fibers, cellophane flakes, chopped rope, cereal grains, mica, synthetic and natural polymers, and the like have been employed as diverting agents.
A myriad of prior art materials used as diverting agents all have at least one or more of the following disadvantages especially when used in conjunction with acidizing of oil-bearing formations: (1) extremely low or slow solubility in formation hydrocarbons, i.e., hydrocarbon oils or gases; (2) excessive solubility in formation hydrocarbons; (3) extremely high costs; (4) random particulate structure having poor packing or sealing characteristics and being difficult to inject into the formation by pumping because of bridging in the well; (5) non-reversibility in sealing or plugging characteristics, i.e., difficulty in removing the plug or bridge; (6) non-selective plugging characteristics; (7) densities lighter than the treating fluid causing the particles to float therein, whereby the particles fail to reach the intended zones; (8) improper size or shape of the diverting agent; and (9) in acid-treating operations the diverting agent may be attacked and dissolved by the acidizing solution.
Thus there exists a need for a diverting agent, particularly for use in acidizing operations, which overcomes the aforementioned deficiencies of these materials.
Accordingly a principal object of this invention is to provide a method of acidizing heterogeneous formations.
Another object is to provide such a method wherein the acidizing solution is diverted from the more permeable portions of the formation to the less permeable portions of the formation during the acidizing treatment.
A further object is to provide such an acidizing solution wherein the diverting agent employed is removed from the oil-producing portions of the formation following the acidizing treatment by dissolving in the oil.
A still further object is to provide an acidizing solution containing a diverting agent which diverting agent tends to predominantly bridge on the face of the formation rather than enter the pores of the formation.
Other objects, advantages and features of the invention will become apparent from the following description and appended claims.